As everyone knows, a 3G core network is comprised of a mobile switching center (“MSC” for short), a visitor location register (“VLR” for short), a home location register (“HLR” for short) and a radio network controller (“RNC” for short), wherein the MSC may be classified into gateway mobile switching center (“GMSC” for short) and visitor mobile switching center (“VMSC” for short) according to functions, the GMSC is responsible for inquiring the MSC of routing information of a called user, and the VMSC is a visited MSC of a user and also a destination that a call is to be routed to; and the VLR is a visitor location register of a user, the HLR is a home location register of a user, and the RNC is a network element responsible for completing radio access function of a user.
When inquiring the routing, a GMSC will first send a routing request message (send routing information, “SRI” for short) to an HLR which then sends a request for providing a roaming number (provide roaming number, “PRN” for short) message to a VLR where a called user is located. After receiving the PRN message, the VLR where the called user is located will check parameters in the message. If the message requests the called VLR to start a pre-paging process, a PRN RSP message will be returned to the HLR after the pre-paging is finished, and a SRI RSP message is returned by the HLR to the GMSC. In this case, if the VMSC supports the pre-paging, a process of pre-paging may last about 10 seconds. Then, during the pre-paging period it is highly possible that the quick move of called MS will cause a pre-paging failure. When the VMSC does not support the pre-paging, though the VMSC will quickly return a PRN response message to the HLR, it is still very possible that the user moves during this process. Such situation still may occur even if the probability is quite low.
The existing roaming retry technology is a technology used for solving the problem of call failure caused by the user moving out of the current location area while the final called user is called. Since there may be a certain period between the time when the HLR has requested and obtained the roaming number from the VMSC of a called user and the time when the VMSC where the user is located receives an initial address message (“IAM” for short) sent from the GMSC, a user having a strong mobility and located at an edge of the location area may move into another location area. Thus, this technology is advantageous for improving the probability of successful call of the user having a strong mobility.
A flow of the existing roaming retry technology is as shown in FIG. 1. FIG. 1 is a schematic diagram of an information interaction of a roaming retry in the related art. As shown in FIG. 1, the flow mainly includes the following processes (step S101-step S117).
Step S101: a GMSC first sends a SRI message to a HLR, wherein the message carries a roaming retry support flag.
Step S102: the HLR then forwards a PRN message to an original VMSC/VLR for requesting a roaming number, wherein the message carries the roaming retry support flag.
Step S103: the original VMSC/VLR returns to the HLR a PRN ACK message carrying a mobile station roaming number (“MSRN” for short).
Step S104: the HLR returns to the GMSC a SRI ACK message carrying the roaming number.
Step S105: the GMSC sends an initial address message (TAM message) to the original VMSC.
Step S106: VMSC/VLR initiates a paging after the original VMSC/VLR receives the TAM message.
Step S107: just at this time, the user moves, and the process of a new VMSC initiating the location updating starts, and a mobile station (“MS” for short) sends a location updating request (Location Update) to the new VMSC/VLR.
Step S108: the new VMSC will send an updating location area request to the HLR.
Step S109: at this time, the HLR sends a canceling location area message (Cancel Location) to the original VMSC which then returns a canceling location area response (Cancel Location Ack) to the HLR.
Step S110: the original VMSC receives the Cancel Location message, and a paging timer may be first stopped to cancel the paging as the paging is still carried out at this time, and the GMSC is informed to perform a roaming retry via step S111; at the same time, the new VMSC/VLR starts a process of inserting subscriber data.
Step S111: the original VMSC/VLR sends a RCH message (Resume Call Handling) to inform the GMSC to perform a roaming retry, which message carries a roaming retry indication identifier.
Step S112: the GMSC releases a call with the original VMSC/VLR using an REL/RLC message (release message of ISUP, corresponding to the IAM message).
Step S113: at this time, after receiving the RCH message, the GMSC will initiate a second process of obtaining a roaming number, which is called roaming retry. The SRI message sent for the second time no longer carries the roaming retry support flag therein.
Step S114: after completing the location updating, the HLR delays sending a PRN to the new VMSC/VLR.
Step S115: after receiving a PRN ACK sent from the new VMSC/VLR, the HLR returns to the GMSC a SRI ACK message containing a new mobile station roaming number (MSRN).
Step S116: the GMSC, using the MSRN′, sends the initial address message (IAM message) to the new VMSC/VLR.
Step S117: the new VMSC/VLR initiates a call process after the location updating is completed.
As seen from the above flow descriptions and various documents describing the protocol in detail, in the current method for triggering a roaming retry, if the user starts to use pre-paging when the original VMSC/VLR receives the SRI/PRN for the first time, and the user moves and receives the canceling location area message (Cancel Location) during the pre-paging, the PRN process at this time will be directly failed if processed according to the previous protocol. And a PRN ACK message carrying an error indication is returned to the HLR in step 103. Also, according to the SDL figure of the revised 3GPP protocol, if the Cancel Location is received during the pre-paging, then the roaming retry during the pre-paging is not taken into consideration. In fact, it is still processed on the basis of call failure.
Thus in the related art, for the situation of non-pre-paging, there is only the solution provided for the problem of roaming retry in the situation of non-pre-paging for the user having a strong mobility. But for the user of pre-paging, the above flow will cause a call failure.